The present invention relates to a semiconductor device.
In semiconductor devices, especially integrated circuit devices using MOS transistors, there is a mounting demand for smaller size and higher speed as the system is being enhanced in function and enlarged in capacity more and more. As a result, a device having a design rule of 1 .mu.m or less has been realized.
The MOS transistor used in such devices is composed of one source, one drain and one gate. As the device size is shrunk, in order to maintain the device characteristics, the source and drain in the device are composed by forming a diffusion layer called LDD, or the gate electrode is made of refractory metals or refractory metal silicide. However, even if the device is further shrunk, the basic structure of the MOS transistor is unchanged, comprising one source, one drain and one gate.
A plan view and a sectional view of a conventional MOS transistor are shown in FIGS. 17A and 17B.
A field oxide film 2 for separating devices is formed on a substrate 1. A device is formed in a region enclosed by the field oxide film 2. The device is composed by forming a source 5a and a drain 5b (positions of 5a and 5b may be interchanged) by introducing impurities of the reverse conductive type of the substrate 1 into the substrate 1. In the region between the source 5a and the drain 5b, a gate oxide film 3 is formed on the substrate 1. A gate electrode 4 is formed on the gate oxide film 3. The structure of only one device is shown here, but in an actual integrated circuit device, a plurality of such devices are disposed, and the desired devices are connected with metal wiring to make up an intended circuit. Meanwhile, an insulation film 6 is formed on the entire surface of the device so as to avoid malfunction due to electric contact by the metal wiring mutually connecting the elements. The metal wiring connecting the devices must contact with the source 5a, drain 5b, or the gate electrode 4. Therefore, specific regions of insulation film 6 are removed by etching, and contact holes 7 for connecting the metal wirings are formed.
There are, however, functions that cannot be realized or hardly realized by the aforesaid construction of the conventional devices, and the development of devices having such functions is being awaited.
One of such desired functions is that, when a specific device is fabricated, a nerve circuit known as neuron should be easily assembled at a similar distribution of device characteristics of other devices manufactured in the same conditions.
In other words, what must be taken into consideration when designing a circuit having a certain function is to design a circuit capable of realizing a high yield in the aspect of fabrication of circuit while operating stably as the designed circuit. That is, it is required to design so as to optimize the circuit operation and to have a sufficient allowance in the circuit operation. As the method for this purpose, for example, it is known to design a basic circuit for optimizing the circuit operation, manufacture the designed device, measure the characteristic of the device, and estimate the characteristic of the intended circuit on the basis of the measured characteristic. In another method, it is known to stimulate the operation of a desired circuit by a computer by using the preliminary extracted parameters of device fabrication and device characteristic.
In both methods, nevertheless, the connecting manners of the transistors, resistors, capacitors and other components of the circuit are determined together with the input and output method and entire circuit construction, and still more the characteristics of the individual elements such as transistors, resistors and capacitors must be predetermined.
The circuit composition may be relatively easily determined by the logic design. On the other hand, the characteristics of individual elements differ depending on the manufacturing conditions, manufacturing equipment, or difference in substrates among manufacturing lots even in identical manufacturing conditions. Accordingly, if a multiplicity of identical devices should be manufactured, discrepancies of characteristics occur among the devices. Therefore, if the allowance of operation of the device characteristics on the whole is discussed by making routine measurement of device characteristics, circuits of high reliability will not be expected. To the contrary, if all device characteristics are carefully examined by either fabricating basic circuits or simulating them, it takes enormous time and cost, and it is not practicable.
Incidentally, the nerve circuit exhibits its effect when it is used to bring the computer more closely to the human thinking manner.
A nerve circuit is composed as a matrix by organically coupling the individual nerve cells. To form such circuit composition by using transistors, the individual prices are intricately combined and the wiring and other structures are very complicated, and it is hard to realize as a circuit.
In order to solve the above problems, the invention intends to present a semiconductor device capable of designing the circuit easily which was difficult in the conventional semiconductor device, and also capable of effectively and easily realizing the formation of the nerve circuit by organically coupling transistors.